1. Field of the Invention
The present invention relates to an image data compression method and an image data processing device implementing the same; more particularly, the invention relates to an image data compression method in which reductions in data amount are achieved by deleting portions of pixel data and to an image data processing device implementing the same.
2. Description of the Related Art
Generally, an image processing system comprises an image data input device for inputting an image to be processed by converting it into digital data, a data processing device for performing processing such as recognition, analysis, manipulation, etc. on the input image data, and an image data output device for outputting the thus processed image data.
Such a system employs a technique of data compression so that sampled image data can be processed efficiently. Data compression is a method of reducing a large volume of original image data into a smaller amount of data. In data compression, it is a condition that an image which is close to an original image can be produced from a small amount of compressed data, and it is desirable that the compression rate be made as large as possible while yet allowing the production of an image as close to the original image as possible.
In one known method of data compression, pixel data that constitute an image are selectively deleted according to a prescribed rule, for example, every other pixel, every two pixels, and so on. If the pixel data are selectively deleted every other pixel, the amount of image data would be reduced to one-half the original data amount and, if the data are deleted for two pixels in every three pixels, the amount of image data would decrease to one-third the original data. In reconstructing the original image from the thus compressed image data, either a neighboring pixel data portion is copied and made to represent a deleted data portion or a technique of interpolation is used in which each deleted portion of data is the average of the pixel data portions neighboring it, assuming that the data changes linearly between them.
However, the above image data compression method that involves deletion of data according to a prescribed rule has the following problems. For example, in the former case in which each deleted portion of pixel data is approximated just by copying the pixel data neighboring on one side of it, if the neighboring pixel data is truncated in an area where the value of pixel data abruptly changes, the information that represents the abrupt change will also be deleted, so that the abrupt change will not be reflected in the reproduced image, thus degrading the quality of reproduction. This is also true of the latter-above case in which deleted pixel data portions are approximated by the pixel data neighboring on both sides thereof assuming that the data changes linearly between them; that is, when the difference between the actual value and the value approximated by the neighboring pixel data is large, the reproduced image quality will degrade. As would be expected, these problems occur where abrupt changes in image data are encountered. In a color image, a change in image data manifests itself in a change in hue, luminosity, and saturation. These changes may sometimes be referred to, collectively, as a color change, and the term "color change" will be used throughout this specification as a term that broadly refers to the change of color including changes in luminosity and saturation.